1. Field of the Invention
The present invention relates to a process for removing sulfur dioxide and fly ash from flue gas originating from large boiler units such as those used in power plants and to a baghouse unit for use in said process.
2. Prior Art
Various processes for reducing the content of sulfur oxides in flue gas are known. A review of such processes appears in the published British patent application No. 2021086A and in U.S. Pat. No. 4,197,278.
The present process is of same type as the one dealt with in said U.S. Pat. No. 4,197,278. In this process sulfur oxides are removed from flue gas in two steps, viz. both a combined spray drying and absorption process, and a subsequent filtering process.
In the former step, an aqueous medium containing a basic absorbent is sprayed into the hot flue gas, whereby the sprayed aqueous medium is dried into a powder while at the same time a chemical reaction takes place between the basic absorbent and the sulfur oxides present in the flue gas, which results in these oxides being bound in the powder, mainly as sulfite.
In the second step of said prior art process, the flue gas is, together with the powder produced during the first step and in suspension in the flue gas, led to a filter, such as a baghouse, where said powder is filtered off together with fly ash, if any. Said powder contains unreacted basic material which, while the powder is in the filter, comes into further contact with the flue gas, which results in some absorption of sulfur oxides taking place also during this second step.
According to said U.S. patent specification, an advantage of the process is that the flue gas is not cooled so much that it needs reheating prior to its discharge into the atmosphere.
In order to achieve optimum conditions for reaction between sulfur oxides and basic absorbent in the spray drying-absorption process and thereby a maximum utilization of the absorbent, it is however suitable to reduce the temperature of the flue gas in this spray drying-absorption process to a value which is lower than the required value, if the flue gas were to be discharged into the atmosphere without reheating. In such case it is necessary to reheat the flue gas in order to ensure that it is sufficiently dispersed into the atmosphere.
This reheating can often be performed most economically and conveniently by admixture of a minor amount of hot, not purified flue gas from the same or similar source as the flue gas which is being purified.
As this non-purified flue gas of course contains sulfur oxides, it is of importance that it should be used in a comparatively small amount, for which reason it should be as hot as possible in order to produce the desired temperature increase. Consequently, in plants where the main portion of flue gas after its discharge from the boiler passes through heat exhanging members, the flue gas to be used for reheating will preferably be removed direct from the boiler plant and by-pass said heat exchange members.
In addition to sulfur oxides, the flue gas used for the reheating contains fly ash which has to be removed before allowing the gas to be discharged into the atmosphere.
It has therefore been obvious to add this hot flue gas serving for reheating to the flue gas coming from the spray drying-absorption process prior to the introduction of the latter into the baghouse, in such an amount that the flue gas after passing through the filter will be sufficiently hot to permit its discharge into the atmosphere. This solution results in an overall temperature increase in the baghouse plant.
The minimum temperature permissible for the purified gas to be released into the atmosphere lies typically between 75.degree. and 95.degree. C. When using the above-mentioned reheating process, a temperature of this order will therefore prevail over the whole filter area.